Digital pulse-position modulation of optical fiber solitons

Amplitude-shift keying modulation of solitons is subject to limited capacity because of Gordon-Haus noise and soliton interaction. It is shown theoretically that the capacity can be increased beyond this limit by recoding of the modulation. The design strategy is to make the pulse train strongly interacting, which reduces the Gordon-Haus noise, and to apply subband coding to counter the effects of the information dispersion caused by the strong intersoliton interaction; the subband-coded complex is pulse-position modulated onto the soliton stream. An example shows a 2.35-times improvement in capacity at the transatlantic distance of 6000 km with this method.